Like other traveling salesman, the equipment salesman can spawn randomly in your current sector as one of the available player events. He will announce himself the usual way and remain in the sector for 15 minutes. Among the usual equipment, he will have the XSTN-IV main quest item for sale for 3 million credits. In order to buy the XSTN-IV module you will need to be allied with the faction that the Equipment Salesman belongs to.
Random events occur once every 12 minutes or so. There are five event types to choose from and so a 20% chance of a spawntravelingmerchant event. There are then four kinds of merchants, after which the mobile equipment merchant has a 50% chance of carrying the XSTN-IV in his inventory. This yields a 2.5% chance of the artifact being available per event, for an expected value of once every eight hours of playtime.
As an alternative to waiting for the random event to occur, you can purchase a Trade Guild Beacon with the "Rare Artifacts" option from an Equipment Dock. The beacon will summon a merchant carrying the artifact, among other goods.